Evaporation boats for use in vapor deposition

ABSTRACT

The evaporation boat comprises superposed upper and lower sheets. The upper sheet includes a upwardly convexed central section and a pair of side sections bent back toward the central section and the lower sheet includes a downwardly convexed central section and a pair of side sections bent inwardly toward the upper sheet. The upper and lower sheets are superposed each other such that the central sections of the upper and lower sheets define a substantially closed chamber for accomodating source material to be evaporated, that slit shaped vapor passages are formed between the upper and lower sheets or the opposite sides of the chamber and that any straight line drawn between any point on the surface of the source material and any point in the passages intersects the inner wall of the upper or lower sheets.

BACKGROUND OF THE INVENTION

This invention relates to an evaporation boat, and more particularly toan evaporation boat for evaporating material to form a thin film of thematerial on a substrate by vacuum deposition technique wherein there isa tendency of depositing particles larger than 1 micron.

Vacuum deposition technique, for example, is used to form aphotoconductive film, optical stripe filters, dichroic mirrors or thelike which constitute a tanget for a pick up tube.

As the evaporation sources for vacuum deposition are used a crucible,coil or boat depending upon the field of application. Among thesevarious types of the evaporation source, evaporation boats are used mostwidely because of its cheapness and easiness in handling. Most of theboats have a simple construction utilizing a thin sheet made of a metalhaving a high melting point, tungsten, molybdenum and tantalum, forexample, and provided with a recess. The longitudinal opposite ends ofthe boat are connected to an electric source to heat and evaporatevapourizable material contained in the recess.

With a boat of such simple construction when the material to beevaporated comprises a sublimatable material, solid particles having adiameter of several microns also formed together with the vapour of thematerial.

When such solid particles having a diameter of larger than 1 micron arecontained in the vapour deposited films, the quality thereof would begreatly degraded. For example, the light receiving area of SATICON tube(trade mark) is 12.7 mm × 9.5 mm for a tube diameter of 1 inch, and 8.8mm × 6.5 mm for a tube diameter of 2/3 inch. In an image monitor, anenlarged image, for example 14 × 21 inches, is formed on an imagereceiving tube. Accordingly, should the photoconductive film of an imagepick up tube contain massive foreign matters, picture defects in theform of dark spots or bright spots, reproduced on the screen of an imagereceiving tube, would be formed thus greatly degrading the quality ofthe picture.

To eliminate this defect, a so-called laminated type evaporation boat asshown in FIG. 1 has been proposed. As shown in FIGS. 1A and 1B, the boat11 comprises upper and lower recessed sheets 14 and 12 and anintermediate partition sheet 13. The intermediate partition sheet 13 isprovided with two spaced perforations 13a while the upper sheet a singleperforation 14a at a point intermediate of said perforations 13a. Thematerial to be evaporated 15 is contained in the recess of the lowersheet 13. When current is passed through the boat by connecting itacross a source of supply the material 15 is evaporated by the heat ofthe boat and the vapour of the material is ejected through tortuouspassages including perforations 13a and 14a. Solid particles having adiameter larger than 1 micron are removed by the collision thereofagainst the inner wall of the upper sheet 14.

With this laminated type evaporation boat as the vapour much pass twicethrough perforations thus decreasing the vapour deposition speed. Inaddition, this construction requires higher temperature for the boat.

SUMMARY OF THE INVENTION

Accordingly, it is an object of this invention to provide an improvedevaporation boat that can prevent solid massive matters from depositingonto the surface of a substrate.

Another object of this invention is to provide an improved evaporationboat capable of preventing solid massive matters from depositing ontothe substrate without any appreciable decrease in the vapour depositionspeed.

According to this invention these and further objects can beaccomplished by providing an evaporation boat comprising superposedupper and lower sheets, the upper sheet having a upwardly convexedcentral section and a pair of side sections connected to the transverseopposite ends of the central section and bent back toward the centralsection, the lower sheet having a downwardly convexed central sectionand a pair of side sections connected to the transverse opposite ends ofthe downwardly convexed central section and bent inwardly toward theupper sheet, and the upper and lower sheets being superposed each othersuch that the central sections of the upper and lower sheets define asubstantially closed chamber for accomodating source material to beevaporated, that slit shaped vapour passages are formed between theupper and lower sheets on the opposite sides of the chamber and that anystright line drawn between any point on the upper surface of the sourcematerial contained in the chamber and any point in the passagesintersects the inner wall of the upper or lower sheet.

According to a modified embodiment flat intermediate sections areinterposed between the opposite ends of the central section and the bentback side sections of the upper sheet for forming relatively long slitshaped vapour passages.

According to another modification the slit shaped vapour passages aremade to be tortuous for more accurating presenting escape of particles.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1A is a longitudinal sectional view showing one example of a priorart evaporation boat;

FIG. 1B is a cross-sectional view taken along a line 1B--1B shown inFIG. 1A;

FIG. 2 is a graph showing a primary particle diameter distribution ofeach lot of the evaporation sources;

FIG. 3 is a graph showing the relationship between the number of theparticles deposited on the surface of a substrate and the particlediameter;

FIG. 4 is a graph showing the relationship between the deposition speedto the substrate and the number of particles deposited on the surface ofthe substrate;

FIG. 5A is a longitudinal sectional view of one embodiment of theevaporation boat constructed according to the teaching of thisinvention;

FIG. 5B is a cross-sectional view taken along a line 5B--5B in FIG. 5A;and FIGS. 6 and 7 are cross-sectional views showing modified embodimentsof this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To have a better understanding of this invention, the behaviour ofextremely small particles contained in metal vapour will firstly beconsidered. In the case of a sublimating material, the materialevaporates before melting so that the primary particles comprising apowder or a sintered mass utilized as the evaporation source fly or theperipheral portions of the primary particles sublimate with the resultthat particles having smaller diameters than the primary particles willfly. Such flying is caused by the fact that the solid particles areblown upwardly by the molecular vapour of the material and such flyingparticles deposit on the surface of a substrate at a certain probabilitythus forming a defective vapour deposited film, where depositedparticles hereinafter refer to as "substrate deposit particles".

When vapour depositing a sublimating material, different fromnon-sublimating material wherein larger secondary particles are formedby the progress of sintering or fusing of the primary particles duringheating, the sublimating material begins to sublimate by the radiationheat from the boat, whereby the bondings between the primary particlesare broken.

This fact was clarified by the following experiment.

That is, in the experiment three lots of different types of cerium oxidepowder having different particle diameters were used as an evaporationsource. FIG. 2 is a graph showing a primary particle diameterdistribution of each lot of the above evaporation sources.

In FIG. 2, curves A and B show commercial cerium oxide lots, which curveC a specially prepared cerium oxide lot in which primary particleshaving diameters of smaller than 5 microns were removed by sintering andclassification. In this experiment, a powder of cerium oxide was placedin a V type molybdenum boat under a vacuum of lower than 5 × 10⁻ 5 torr,the deposition speed of the cerium oxide vapour to the surface of thesubstrate was limited to 0.4 Angstrom/sec. and the substrate was spaced30 cm from the boat thus forming a cerium oxide film having a thicknessof 1000A.

FIG. 3 is a graph showing the relationship between the number of theparticles deposited on a unit area of the substrate and the particlediameter when materials corresponding to curves A, B and C were vapourdeposited. As shown, curves A' and B' are generally similar to curves Aand B shown in FIG. 2, but curve C' is not similar to curve C showingthat the deposition of substrate deposit particles having particlediameters less than 5 microns was prevented.

From this it was concluded that the fly of the primary particles isprobably caused by the energy exchange between the vapour and theparticles by the collision of the primary particles during flying. Thesame conclusion can also be made from a graph shown in FIG. 4 showingthe relationship between the average number of particles depositing on aunit area of the substrate and the deposition speed which is expressedin terms the amount of energy of the vapour molecules. Moreparticularly, in FIG. 4, a straight line X depicted on a logarithmicsection paper represents the number of particles deposited on thesubstrate and having a particle diameter of from 1 to 5 microns. Thestraight line Y shows the number of the deposited particles having adiameter of from 5 to 10 microns, whereas straight line Z the number ofthe deposited particles having a diameter of larger than 10 microns.FIG. 4 shows that particles having a small imparted energy are difficultto fly and that large particles are also difficult to fly, thus showingthe manner of flying of the primary particles.

This invention utilizes such flying behaviour of the particles, and theobjects of the invention can be accomplished when the followingconditions are satisfied.

Firstly, it is necessary to remove minute primary particles in the boatsince they are always contained in the evaporation source material. Inother words, it is necessary to sinter and fuse together minuteparticles in the boat thereby forming harmless larger particles. To thisend, it is necessary to totally close the boat.

Secondly, as the primary particles tend to fly out from the surfaces ofthe evaporation source material it is necessary to make the temperatureof the surface of the source material to be equal or slightly higherthan the maximum temperature of the source material to cause the surfaceparticles to sinter or fuse at an earlier time than the particles atother portions. To this end, it is necessary to make the temperature ofthe wall of the boat in contact with the source material to be the sameor higher than the temperature of the other portions of the boat. Tothis end, the boat must be of a closed type having a higher walltemperature wherein the boat is surrounded by the heat generating unitsmade of the same material and having the same dimension, or surroundedby heat generating units made of different materials or having differentdimensions.

Thirdly, where a closed type boat is used it is necessary to prevent alarge decrease in the deposition speed. To this end, it is necessary tonarrow the vapour exit opening of the boat by forming a narrow and longslit.

Fourthly, it is necessary to construct the boat such that flyingparticles come to collide one or more against the wall of the boat thusloosing their kinetic energy, or that the particles are prevented fromflying toward the substrate.

To accomplish this object the boat much be constructed such that theexit opening is not formed along any straight lines interconnectingrespective points on the surface of the source material and thesubstrate, or that a deep vertical slit is formed. With such a slit asthere is no vapour in the slit at the time of starting the heating, noheat energy is imparted to the vapour so that the wall of the slitassume higher temperature than the other portions of the boat.Accordingly, the vapour of the source material does not deposit on thewall of the slit. Even when deposition occurs, the deposited materialwill evaporate at once and force back the vapour approaching the slit.Due to repeated reevaporation and forcing back at the slit the size ofthe flying particles is greatly decreased. Moreover, as no particle isformed at the time of reevaporation, substantially no particle can passthrough the slit. No material was deposited on the wall of the slitafter a vapour deposition operation.

Fifthly, the exit opening should be shaped such that it guides themolecular vapour of the source material toward the substrate when thevapour comes to the outside of the boat through the exit opening.

Sixthly, the boat should comprise upper and lower units. Thisconstruction assures easy loading of the source material into the boatas well as easy fabrication of the boat.

The following examples of the boat were constructed to satisfy theconditions described above.

EXAMPLE 1

In the example shown in FIGS. 5A and 5B the boat generatly designated bya reference numeral numeral 20 comprises a upper sheet 22 and a lowersheet 21 having the same dimension and manufactured by the same heatgenerating material. As shown in FIG. 5B, the cross-section at thecentral portion of the lower sheet 21 has a V shape which is downwardlyconvexed. The transverse opposite sides of the sheet are bent upwardlyin symmetric relation with respect to the bottom or heel 23 of therecess. The upper portions 25a and 25b of the sides 24a and 25a of thesheet are bent inwardly.

The sides 27a and 27b of the central portion of the upper sheet 22 isalso bent to form an inverted V above the recess of the lower sheet 21,the heel 26 of the recess of the upper sheet vertically aligning withthe heel 23 of the lower recess. The upper sheet 22 with both sides bentcomprises a lid for the lower recess. Portions 28a and 28b on theoutside of the sides 27a and 27b are bent back toward the sides. Therecessed upper and lower sheets 22 and 21 define a chamber 31 forloading the evaporation source material 30. The upper and lower sheetsare overlapped to form slits 35a and 35b of a definite width between thebends 32 and 33 of the upper sheet and the lower sheet. The bentportions 28a and 28b of the upper sheet and the inwardly bent portions25a and 25b of the lower sheet 21 cooperate to form V shaped vapourejection openings 36a and 36b. It is to be noted that any straight linebetween any points on the surface of the source material 30 charged inthe evaporation chamber 31 and any point in the slits 35a and 35bintersects the inner surface of the upper sheet 22 or the inner surfaceof the lower sheet 21. The opposite ends of the upper and lower sheets22 and 21 are united into integral extensions as shown in FIG. 5A andthe extensions are damped by annular metal bands 38 and 39 made ofmolybdenum, for example.

A powder of cerium oxide having a characteristic as shown by B of FIG. 2was loaded in the boat, and electrically heated at under a vacuum oflower than 5 × 10⁻ 5 torr to form a vapour deposited film having athickness of 1000A. The film was compared with a film prepared withoutusing the upper sheet 22 and obtained the result as shown in thefollowing table 1.

                  Table 1                                                         ______________________________________                                                Number of particles      Number of                                            on the substrate                                                                             Percentage                                                                              particles                                            having an average                                                                            of        in the boat                                  Type of particle       good      having a dia-                                the boat                                                                              Size of 1 to 5μ*                                                                          substrate of 1 to 5 μ                               ______________________________________                                        example 1                                                                             0.5/substrate  80 %      ≦2.0×10.sup.4 /cm.sup.2         no upper                                                                              5.0/substrate  20 %       1.2×10.sup.7 /cm.sup.2                sheet                                                                         ______________________________________                                         *Mean value obtained by measuring one hundred substrates each having 1.5      cm.sup.2 area                                                            

As can be noted from Table 1, according to this invention, since thenumber of the particles in the boat is reduced greatly, the number ofthe particles deposited on the substrate is also reduced by one order ofmagnitude with the result that the quality of the deposited film isimproved greatly.

This is caused by the face that nearly all particles contained in thevapour in the boat are arrested by the inner surface of the upper sheet16 and particles that have escaped through the slits 35a and 35b arearrested by the inwardly bent portions 25a and 25b.

Consequently, only the vapour of the source material can reach thesubstrate.

EXAMPLE 2

FIG. 6 shows a second embodiment of this invention in which portionscorresponding to those shown in FIGS. 5A and 5B are designated by thesame reference numerals. In this embodiment, between sections 27a and28a and between sections 27b and 28b of the upper sheet 22 areinterposed flat sections 40a and 40b which are paralled with sections25a and 25b of the lower sheet 21 to form narrow slits 35a and 35btherebetween. Since these slits present substantial resistance to thevapour passing through the slits, the pressure in the loading chamber 31is increased thereby enhancing the sintering or fusing of the sourcematerial 30. As a result, discharge of the particles through the slit isdecreased.

EXAMPLE 3

FIG. 7 shows still further modification of this invention wherein thelower sheet 21 is formed with a semicircular bottom 45 at its center,horizontal sections 46a and 46b on the opposite sides of the bottom 45,and bent up sections 47a and 47b on the opposite ends of the horizontalsections 46a and 46b. Ribs 48a and 48b are formed at substantially thecenters of respective horizontal sections 46a and 46b. The upper sheet21 is formed with semicircular central section 51 confronting thesemicircular bottom 45 of the lower sheet 21, horizontal sections 52aand 52b facing the horizontal sections 46a and 46b of the lower sheetand bent back sections 53a and 53b on the opposite ends of thehorizontal sections 52a and 52b. The horizontal sections 52a and 52b,and 46a and 46b are spaced a predetermined distance and provided withlongitudinal ribs 54a, 54b, 48a and 48b respectively which are alsospaced each other. Accordingly, relatively long slit like vapourpassages 56a and 56b are formed between the loading chamber 57 and Vshaped ejecting ports 55a and 55b.

In this embodiment, in as much as the passages 56a and 56b are tortuousthey are effective to increase the pressure in the loading chamber 57.Any straight line drawn between any point on the upper surface of thesource material 30 and any point in the passages 35a and 35b intersectsthe inner surface of the upper sheet 22 or the low sheet 21 and sincethe passages 56a and 56b are tortous, the particle tending to passthrough the passages are more effectively arrested.

The boats of Examples 2 and 3 were used to form films of germanium oxidehaving a thickness of 100A under the same condition as in Example 1 andobtained excellent result as shown in the following Table II.

It was found that these boats are also effectives for the vapourdeposition of alloys of the As-Te type. With As-Te alloys, containingmore than 50% of Te, although the alloys melt in the boat due to abruptboiling of As, the particles tends to fly. For this reason, it has beendifficult to form films of good quality with the boat of the other type,but when the boat of this invention is used it is possible to readilyform vapour deposited films of high quality.

                  Table II                                                        ______________________________________                                                Number of particles      Number of                                            on the substrate                                                                             Percentage                                                                              particles                                            having an average                                                                            of        in the boat                                  Type of particle       good      having a dia.                                the boat                                                                              size of 1 to 5μ*                                                                          substrate of 1 to 5μ                                ______________________________________                                        FIG. 6   0.1/substrate 80 - 100 %                                                                              <2.0×10.sup.4 /cm.sup.2                FIG. 7  <0.1/substrate 90 - 100 %                                             ______________________________________                                        ≢*Mean value obtained by measuring one hundred substrates each   

As can be noted from the foregoing description the invention provides animproved evaporation boat which is of the closed type, provided withslit shaped vapour passages and constructed such that particlescontained in the vapour of the source material do not directly impingeupon the substrate to be vapour deposited. Accordingly, it is possibleto form vapour deposited films of high quality.

While the invention has been shown and described in terms of somepreferred embodiment thereof it will be clear that many changes andmodifications will be obvious to those skilled in the art withoutdeparting from the scope of the invention as defined in the appendedclaims. For example, although the boat was made of molybdenum any highmelting point metals and alloys such as tungsten, tantalum, etc, canalso be used.

What is claimed is:
 1. An evaporation boat comprising superposed upperand lower sheets, said upper sheet having a upwardly convexed centralsection and a pair of side sections connected to the transverse oppositeends of said central section and bent back toward said central section,said lower sheet having a downwardly convexed central section and a pairof side sections connection to the transverse opposite ends of saiddownwardly convexed central section and bent inwardly toward said uppersheet and said upper and lower sheets being superposed each other suchthat said central sections of said upper and lower sheets define asubstantially closed chamber for accomodating source material to beevaporated, that slit shaped vapour passages are formed between saidupper and lower sheets on the opposite sides of said chamber and thatany straight line drawn between any point on the upper surface of thesource material contained in said chamber and any point in said passagesintersects the inner wall of said upper or lower sheet.
 2. Theevaporation boat according to claim 1 wherein said bent back sidesections of the upper sheet and said side sections of said lower sheetdefine form V shaped vapour ejecting ports respectively connected to theexit ends of said slit shaped vapour passages.
 3. The evaporation boataccording to claim 1 wherein the central section of the upper sheettakes the form of an inverted V and that of the lower sheet takes theform of an V.
 4. The evaporation boat according to claim 1 wherein saidslit shaped vapour passages are formed between the edges at which saidside sections of the upper sheet are bent back and the edges at whichsaid side sections of said lower section are bent.
 5. The evaporationboat according to claim 1 wherein the central section of said uppersheet takes the form of a upwardly curved semicircle whereas the centralsection of said lower sheet takes the form of a downwardly curvedsemicircle.
 6. The evaporation boat according to claim 1 wherein saidupper sheet further includes intermediate sections which are disposedbetween the transverse opposite ends of said central section and saidbent back side sections respectively, and said intermediate sections aredisposed in parallel with said inwardly bent side sections of said lowersheet to define therebetween relatively long slit like vapour passages.7. The evaporation boat according to claim 5 wherein said upper sheetfurther includes horizontal intermediate sections between saidsemicircular central section and said bent back side sectionsrespectively, and said lower sheet further includes horizontalintermediate sections between said semicircular central section and saidinwardly bent side sections, said horizontal intermediate sections ofsaid upper and lower sheets defining therebetween relatively long slitlike vapour passages.
 8. The evaporation boat according to claim 7wherein each of said horizontal intermediate sections of said upper andlower sheets is provided with a longitudinal rib thus forming tortuousvapour passages.